EN
The Engineering Link Between Speed Governors and Fuel Consumption
How engine load, aerodynamic drag, and combustion efficiency scale with speed
Diesel engine fuel consumption depends on three interrelated factors dictated by the speed of the vehicle. First, as the speed of the vehicle increases, a phenomenon known as aerodynamic drag becomes significant. This is a result of vehicle speed. When speed doubles, aerodynamic resistance increases by a factor of four. Air resistance acts as a significant drain on energy-budget above a speed of 55 mph. Second, the optimal engine efficiency is around 1200-1800 revolutions per minute. Fuel, air, and turbocharger pressure conditions provide the best combustion and pressure management. Below 40 mph, the engine achieves a state of poor combustion, producing soot and hydrocarbons. Third, above 70 mph the engine becomes inefficient and the energy spent overcoming vehicle friction becomes significant.
The quantity of work performed by an engine varies based on how far the accelerator pedal is depressed and how much fuel is injected into each cylinder. These factors are governed by governors that aim to control things with a view to minimising excessive mechanical stress and thermal problems. For instance, if a truck has a top speed limit of 65, rather than a maximum of 75 mph, the engine has roughly a third less aerodynamic resistance from the air and can operate at peak efficiency for longer.
What's so Special About the Fuel Efficiency Sweet Spot for Diesel Powertrains?
Engaging the diesel engine fuel economy sweet spot of 50 to 65 miles per hour yields the best “brake specific fuel consumption” (BSFC) outcome for the engine. It yields the best fuel consumption outcome by creating the best thermal balance inside the engine, optimal air stream around the truck body, and efficiency of the driveline. At speeds under 50 mph, the diesel engine falls out of its optimal power band, and the driveline uses lower gears, which causes increased friction losses. At speeds greater than 65 mph, fuel economy suffers because of increased aerodynamic drag, which at 70 mph accounts for approximately two-thirds of the power needed to maintain that speed. This is why diesel powered vehicles obtain the optimum fuel consumption outcome by operating in this ‘middle ground’ as the economy fuel sweet spot.
Turbochargers offer continuous and efficient boost of 15–25 psi
High pressure common-rail injectors work around the limits of volumetric efficiency
Rolling resistance remains mostly unchanged
Transmissions allow continuous operation in the top gear at 1200–1800 RPM
This convergence allows for a 30% improvement in fuel economy compared to unrestricted operation at 75 mph. Speed governors reliably enforce this, particularly for heavy-duty vehicles, which have drag coefficients over 0.65 due to their boxy shape and trailer configuration.
The Impact of Various Speed Governors on Real-World Fuel Efficiency
The positioning of speed governors can either be a hard or soft limit and can therefore either be constraining or relaxing regarding throttle position and how the ECU manages fuel flow
The fuel delivery for the engine is completely closed off when hard limiters engage, which results in the of the throttle the driver may feel, and becomes especially evident when the vehicle is being operated on the highway. This sudden fuel cutoff disruption affects engine stability resulting in the vehicle inefficiently expending more fuel. Fuel efficiencies may be adversely affected by as much as 12% to 8% when compared to a system which operates as designed. This is exactly where soft limiters function differently. These systems utilize a mechanism where the ECU is predictively fuel mapped to arrest a fuel increase. This mechanism retains the vehicle's operational integrity and fuel efficiency when compared to offensive limits during passing maneuvers and defensively maintaining fuel efficiency during aggressive acceleration to reduce the overall engine speed.
Torque demand, road grade, and payload data for adaptive speed governor tuning.
As an example, modern governor systems use IMUs and axle load data to adjust speed limits dynamically based on the torque request for the vehicle. These smart systems, for example, know that when traversing a 5% grade uphill, they should extend the duration of a particular gear to minimize excessive downshifting and engine overspeed. Fleet operators have expressed empirically observable systems engagement in the inverse: based on the load that the trucks are carrying, the governor system will reduce the maximum speed permitted. After analyzing telematics data from several major North American trucking fleets, this approach was seen to reduce overall fuel consumption by 3.1 gallons for every 1,000 miles driven. In contrast, the traditional approach of imposing a speed limit for a given road segment based on historical data, regardless of the segment's grade or the load carried by the truck, is excessively simplistic. These new adaptive systems have transformed speed control from a simplistic approach to dynamic performance needs based on actual field conditions.Fuel Savings from Speed Governor Use.
Calibrated speed governors have proven that fuel savings are attainable during commercial fleet operations. Controlling speeds to a range of diesel fuel efficiency (50 – 65 mph) allows for a reduction of fuel consumption by 10 – 15% operationally unrestrained driving. These savings are a result of both reduction in aerodynamic drag and stabilized combustion.
Fuel savings from the use of speed governors for commercial fleets can be attributed to the following:
- Dra g savings: the faster the driving speed the more significant the fuel savings [within the range of 50-65mph].
- Maintaining a Steady State: Maintaining a controlled driving speed prevents changes in throttle position and preserves the optimal timing for the fuel injector to pump the fuel, and optimal response of the turbo.
- Fleets of 100 trucks averaging 100,000 miles, fuel savings of 150,000 gallons of diesel annually can be achieved. When used in conjunction with driver training and route optimization, the savings can be achieved without increasing the time, and with a reduction in CO₂ emissions.
Beyond Basic Limiting: Intelligent Speed Assistance as the Advanced Form of Speed Governor
From reactive speed control to predictive eco-cruise powered by GNSS, HD maps, and V2X
Intelligent Speed Assistance is a predictive alternative to the old fashioned speed governors and their reactive modes of operation. Old fashioned governors intervene only AFTER speed limits have been crossed, and they do so in a disruptive and inefficient way, causing sudden reductions in power and speed fluctuations. The Intelligent Speed Assistance system is able to do predictive eco cruising thanks to the integration of GNSS, detailed road maps, and vehicle to infrastructure communications. Predictive eco cruising allows these systems to have a proactive stance, so they can anticipate obstacles in the road as a function of the terrain (hills, curves), road traffic, and speed limits up to 3km. This allows for optimal control of power to the wheels and prevents problems rather than reacting to them.
Acceleration Algorithms, Adaptive cruise control, and Integrated traffic control systems, result in the flattening of vehicle speed profiles and the optimization of the entire driving cycle from an energy consumption perspective. The result of the combination of these technologies is an 15-20% reduction in speed variance that is traditionally considered to be fuel wasting from these reactive systems, and an increase of fuel conserving technologies that utilize an intelligent approach, instead of the mechanical building block approach of simply using a speed limiter.
Frequently Asked Questions (FAQ)
What are the factors that influence fuel consumption in diesel engines?
These variables are dependent on a combination of vehicle speed, aerodynamic drag, and engine RPM. The relationship of these variables is that at great speeds, aerodynamic drag increases exponentially, while at low speeds, engine RPM can result in greater efficiency.
What is the reason the 50–65 mph range is considered the fuel efficiency sweet spot for diesel engines?
It is at this speed range that there is a perfect balance between the engine and the mechanical components of the driveline for optimum fuel economy.
What are hard and soft limits in relation to speed governors?
Hard limits result in sudden cuts to fuelling, causing erratic engine behaviour and losses in fuel economy, while soft limits are able to optimise fuel input and ensure sustained operation at an efficient level with minimal fuel losses, as they predictively map fuel changes.
In what ways are adaptive speed governors able to enhance fuel economy?
The adaptive type of system alters its speed limits in relation to changing road conditions and the weight of the payload, which allows for an optimal and tailored system performance and less fuel wastage as it corresponds to the required power needs of the vehicle.
What is Intelligent Speed Assistance (ISA), and how is it different from traditional speed governors?
ISA combines control of speed with fuel-wasting event avoidance and overall improved energy efficiency beyond the mere limiting of speed with the use of advanced technologies such as foreign maps, satellite positioning, and vehicle to vehicle communications.
